Friday, March 20, 2015

Spring 2015

When does spring begin?
Astronomically speaking, the March equinox occurs when the Sun crosses the celestial equator on its way north along the ecliptic. In the Northern Hemisphere, the March equinox is known as the vernal, or spring, equinox, and marks the start of the spring season.

In the Southern Hemisphere, this equinox is known as the autumnal, or fall, equinox and marks the start of the fall season; the vernal equinox for the Southern Hemisphere occurs in September.

The March equinox happens at the same moment across the world but is converted to local time. In 2015, it falls on March 20 at 6:45 P.M. EDT, 5:45 P.M. CDT, 4:45 P.M. MDT, and 3:45 P.M. PDT, for example.

Meteorologically speaking, however, in the Northern Hemisphere the official spring season always begins on March 1 and continues through May 31. Summer begins on June 1; autumn, September 1; and winter, December 1.

Weather scientists divide the year into quarters this way to make it easier to compare seasonal and monthly statistics from one year to the next. The meteorological seasons are based on annual temperature cycles rather than on the position of Earth in relation to the Sun, and they more closely follow the Gregorian calendar. Using the dates of the astronomical equinoxes and solstices for the seasons would present a statistical problem because these dates can vary slightly each year.

The Vernal Equinox
Ah, spring! This season brings increasing daylight, warming temperatures, and the rebirth of flora and fauna.

The word equinox is derived from the Latin words meaning “equal night.” All over the world, days and nights are approximately equal. Today, the Sun rises exactly in the east and sets exactly in the west.

At the equinoxes, the tilt of Earth relative to the Sun is zero, which means that Earth’s axis neither points toward nor away from the Sun. (However, the tilt of Earth relative to its plane of orbit, called the ecliptic plane, is always about 23.5 degrees.)

As this season begins, the length of daylight continues to increase since the start of winter. See your local Sun rise and set times—and how the day length changes!

Equinox Facts
Question: Why doesn’t the vernal equinox (equal night) on March 20 have the same number of hours for day and night?

Answer: Our former astronomer, George Greenstein, had this to say: "There are two reasons. First, light rays from the Sun are bent by the Earth's atmosphere. (This is why the Sun appears squashed when it sets.) They are bent in such a way that we are actually able to see the Sun before it rises and after it sets. The second reason is that daytime begins the moment any part of the Sun is over the horizon, and it is not over until the last part of the Sun has set. If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights.’”

Signs of Spring
The vernal equinox signals the beginning of nature’s renewal in the Northern Hemisphere. Worms begin to emerge from the earth, ladybugs land on screen doors, green buds appear, birds chirp, and flowers begin to bloom.